Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have utility as adsorbent materials and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain zeolitic materials are ordered, porous crystalline metallosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO4 tetrahedra and optionally tetrahedra of a Periodic Table Group IIIA element oxide, e.g., AlO4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-1 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983); ZSM-35 (U.S. Pat. No. 4,016,245); zeolite ZSM-23 (U.S. Pat. No. 4,076,842); zeolite MCM-22 (U.S. Pat. No. 4,954,325); and zeolite MCM-35 (U.S. Pat. No. 4,981,663), merely to name a few.
U.S. Pat. No. 6,471,939 describes the synthesis and structure of a zeolite designated ITQ-12, which is a single crystalline phase material having a 2-dimensional channel system comprising two sets of intersecting channels wherein each channel is defined by an 8-membered ring of tetrahedrally coordinated atoms. One set of 8-membered ring channels each has cross-sectional dimensions of about 3.4 Angstrom by about 4.6 Angstrom, whereas the other set of 8-membered ring channels each has cross-sectional dimensions of about 2.7 Angstrom by about 4.8 Angstrom. By virtue of its small pore size, ITQ-12 has shown promise as an adsorbent for propane/propene separation (Olson, D. H.; Yang, X.; Camblor, M. A. J. Phys. Chem. B 2004, 108, 11044-11048).
U.S. Pat. No. 6,471,939 discloses that ITQ-12 can be synthesized in a fluoride medium employing the 1,3,5-trimethylimidazolium cation, and preferably 1,3,5-trimethylimidazolium hydroxide, as an organic directing agent. The organic directing agent is prepared by the dimethylation of 4(5)-methylimidazole with excess methyl iodide in the presence of the inorganic base K2CO3 to neutralize the proton released by the reaction. The reaction is conducted in the polar, aprotic solvent chloroform so that the inorganic salts, KHCO3 and K2CO3, remain insoluble whereas the organic salts, trimethylimidazole iodide, are soluble. However, chloroform and similar halogenated polar, aprotic solvents are classified as hazardous substances and dimethylation reactions are typically much slower than monomethylation reactions.
According to the present invention, it has now been found that ITQ-12 can be synthesized in the presence of polymethylated pyridinium cations, such as 1,2,4,6-tetramethylpyridinium cations, as an organic directing agent. Since these materials can generally be produced by monomethylation of the nitrogen atom of commercially available methylpyridine compounds, the reaction is relatively fast, does not involve the release of a proton and so does not require the presence of an inorganic base and can be conducted in benign solvents, such as ethanol.